A magnetic resonance imaging apparatus is an imaging apparatus configured to magnetically excite nuclear spin of a patient placed in a static magnetic field with an RF (Radio Frequency) pulse having the Larmor frequency and reconstruct an image based on magnetic resonance signals generated due to the excitation.
In many magnetic resonance imaging apparatuses, a superconducting magnet is used to generate a static magnetic field. In a superconducting magnet, a superconducting coil is placed in a liquid helium container and this liquid helium container is surrounded by a vacuum container called a cryostat. A magnetic resonance imaging apparatus is equipped with a helium cryocooler (hereinafter, simply referred to as a cryocooler), which is disposed on a top part of a gantry, for example. The cryocooler cools down the cryostat with a refrigeration cycle of a piston operation inside a cylinder of the cryocooler so that a superconductive state of the superconducting coil inside the liquid helium container is kept.
The piston of the cryocooler reciprocates, for example, with a period of one second, and this mechanical vibration causes periodic fluctuation in a static magnetic field. The periodic fluctuation in the static magnetic field becomes a factor of artifact such as a ghost. In order to suppress such artifact caused by mechanical fluctuation of a cryocooler, a magnetic resonance imaging apparatus further equipped with a corrective magnetic field coil for canceling the effect of periodic fluctuation in a static magnetic field is known.
However, such configuration is forced to include the corrective magnetic field coil for canceling the effect of periodic fluctuation in a static magnetic field in addition to a conventional gradient coil, which becomes a factor of increasing manufacturing cost. Further, adding the corrective magnetic field coil to an existing magnetic resonance imaging apparatus necessitates enormous amount of modification and, and thus it is not realistic.